The present disclosure is directed to a shroud attachment which may be used in a turbine section of a gas turbine engine.
Ceramic materials have been studied for application to components in the hot section of gas turbine engines to replace metallic materials that require substantial cooling in order to withstand the high temperature of combustion gases. Ceramics have been made into turbine blades and vanes and integrally bladed rotors. In these cases, particularly that of ceramic integrally bladed rotors, a large gap between the rotor blade tip and metal shrouds may result from the low thermal expansion of ceramics that made up the blades and the integrally bladed rotors. The low density and high stiffness of ceramics reduce the radial displacement of the blade tip and potentially exacerbate the issue further. The large gap or clearance at the bade tip can result in a high percentage of the core flow leaking through the tip and in so doing, not transferring energy from gas flow to turbine blades, which may cause engine performance penalties as useful energy is not harnessed. The performance penalty can be more severe for small gas turbine engines wherein the small engine dimension makes a small tip clearance large relative to the gas flow path.
Ceramic shrouds have been used to control the gap between rotor blade tip and inner surface of the shroud for ceramic turbines to minimize losses induced by large tip clearance. Due to its high stiffness, low thermal expansion and low thermal conductivity, a ceramic shroud experiences less thermal distortion than a metal shroud for a given set of thermal loading conditions. The high temperature capability of the ceramics also leads to reduced cooling air requirements, an additional benefit to engine performance.
One issue which needs to be dealt with in ceramic shroud design is attachment to the metallic engine structure due to the low ductility and low thermal expansion of ceramics as compared to metals. Elastic springs have been used to support ceramic shrouds. Their performance at elevated temperatures over long durations require monitoring due to metal creep.
Another approach for supporting a ceramic shroud is through the tab and slot approach, where the tabs on the ceramic shroud can slide in and out of slots on a metallic casing. Generally, there are three tab and slot pairs evenly distributed circumferentially to spread the support load and to position the shroud radially. In theory, this approach can minimize thermal constraints by letting the ceramic shroud and metal support grow freely from each other. However, due to manufacturing tolerance control, uneven thermal fields, and thermal deformation of the shroud and the casing, thermal stress at the tabs could be sufficiently high to cause local damage.
Another method to support the ceramic turbine shroud is to use axial tabs that engage partially through axial slots in the shroud. This shroud design is assembled inside a turbine support case, which is often difficult to have easy access and therefore prone to assembly error. Further, the shroud is loaded axially forward from the power turbine vane pack when the engine is in operation. The relative axial movement between the ceramic turbine assembly and the power turbine vane depends on the material thermal expansion and engine conditions and therefore difficult to predict accurately.